This is usually caused by not aligning the top of the first tag properly in pin-feed rollers when you load the tags in to the printer.  If your printer has an automatic load function, you might need to manually roll the first tag back or forward a bit to get the tag to line up properly.  On most dot matrix printers this means having the top of the first tag well down in the rollers.  Once you know where the top of your tags need to be to start, mark or remember the spot so you can rapidly reload tags after tearing them off.  Most printers will 'remember' the sweet spot once you have it set if you use their auto load and tear functions.